Catalyst System for Catalyzed Electrochemical Reactions and Preparation Thereof, Applications and Uses Thereof

ABSTRACT

A catalyst system for catalyzed electrochemical reactions, in particular the electrochemical conversion of carbon dioxide into valuable chemical products, such as carboxylates and carboxylic acids, comprises a catalyst, wherein the catalyst comprises bismuth and indium. The catalyst system can be a component of a gas diffusion electrode, that can be used as the cathode electrode in an electrochemical cell.

The present invention generally relates to a catalyst system forcatalyzed electrochemical reactions, comprising a conductive support anda catalyst, in particular for reducing carbon dioxide in order toprepare products or intermediates thereof like carboxylates and/orcarboxylic acids.

The electrochemical conversion of carbon dioxide into economicallyvaluable materials such as fuels and industrial chemicals orintermediate products thereof is gaining interest in view of mitigatingthe emission of carbon dioxide into the atmosphere, which is responsiblefor climate alterations, changes in pH of seawater and other potentiallydamaging effects like melting of polar ice and sea level rise.

Catalyzed electrochemical reduction of carbon dioxide for preparingeconomically valuable products is known in the art.

E.g. WO2013/006711 discloses methods and systems for the electrochemicalconversion of carbon dioxide to products like carboxylic acids, glycolsand carboxylates in the presence of a homogeneous heterocyclic aminecatalyst. In an embodiment the cathode of the electrochemical cellwherein the conversion is performed, comprises a material suitable forthe reduction of carbon dioxide. Examples of the cathode materialsinclude metal and metal alloys, amongst others indium and indium alloys.

WO2014/032000 discloses a method of reducing carbon dioxide into one ormore organic products in an electrochemical cell, wherein the cathode isan oxidized indium electrode, in particular an anodized indiumelectrode.

WO2014/042781 discloses the electrochemical conversion of carbon dioxideinto products using a high surface area cathode, wherein the cathodeincludes an indium coating and has a void volume of between about 30% to98%. The cathode may also include indium coatings and/or metalstructures further containing Pb, Sn, Hg, Tl, In, Bi, and Cd, theiralloys, and combinations thereof. Metals including Ti, Nb, Cr, Mo, Ag,Cd, Hg, Tl, An, and Pb as well as Cr-Ni-Mo steel alloys among manyothers may be incorporated. The alloys of indium with other metals,including Sn, Pb, Hg, TI, Bi, Cu, and Cd and their mixed alloys andcombinations thereof on the exposed catalytic surfaces of the electrodepreferably comprise 5% to 99% indium.

Preliminary research has indicated that not all the potential catalystsas disclosed in the above prior art documents function as desired interms of selectivity, activity and Faradaic efficiency. Therefore thereis an ongoing need to develop catalyst systems, which show animprovement of one or more of these catalyst properties.

In particular the present invention aims at providing a catalyst systemhaving a high Faradaic efficiency for the electrochemical reduction ofcarbon dioxide and a high selectivity towards valuable reductionproducts, in particular carboxylic acids or intermediates thereof, suchas carboxylate salts.

This invention provides a catalyst system for catalyzed electrochemicalreactions, comprising a catalyst, wherein the catalyst comprises 5-94wt. % bismuth and 6-95 wt. % indium, based on the total amount ofbismuth and indium. This catalyst will herein below be referred to as anindium bismuth catalyst.

The catalyst system according to the invention for catalyzedelectrochemical reactions, in particular reduction of carbon dioxide,preferably comprises an electrically conductive support and a catalyst,wherein the catalyst comprises 5-94 wt. % bismuth and 6-95 wt. % indium,based on the total amount of bismuth and indium.

Surprisingly it has been found that the binary metal combination ofbismuth and indium as catalyst for the electrochemical conversion ofcarbon dioxide shows a good selectivity for the reduction of carbondioxide into carboxylic acids and carboxylates, as well as a goodFaradaic yield, in particular for the aqueous conversion of carbondioxide to formate salt.

Compared to other indium based binary metal catalysts the indium bismuthcatalyst shows an improved Faradaic yield. The amount of bismuth is inthe range of 5-94 wt. % based on the total amount of bismuth and indium,preferably in the range of 10-90 wt. %, more preferably 30-90 wt. %,such as 35-90 wt. %. Experimental results have indicated that an amountof bismuth in the range of 40-60 wt. %, such as 45-55 wt. %, e.g. about1:1 weight ratio of bismuth to indium, offers improved catalyticproperties regarding carbon dioxide to formate conversion.

The catalyst can comprise a combination of bismuth and indium indifferent thermodynamic phases. Preferably an amorphous combination ofbismuth and indium is used. That is, preferably the catalyst systemaccording to the invention comprises a catalyst, wherein the catalystcomprises an amorphous combination of 5-94 wt. % bismuth and 6-95 wt. %indium, based on the total amount of bismuth and indium.

The indium bismuth catalyst according to the invention can be appliedwithout or in combination with an electrically conductive support. Itcan, for example, be applied without or in combination with a carboncontaining support. Preferably the indium bismuth catalyst is applied incombination with an electrically conductive support. Therefore, thecatalyst system is preferably a catalyst system for catalyzedelectrochemical reactions, comprising an electrically conductive supportand a catalyst, wherein the catalyst comprises 5-94 wt. % bismuth and6-95 wt. % indium, based on the total amount of bismuth and indium.

As a conductive support a particulate material, in particular carbonparticles, is used. Preferably the conductive support comprises a porousstructure of carbon particles bonded together. A preferred bindingmaterial is a hydrophobic binder, such as a fluorinated binder. Thecatalyst is deposited onto or adhered to the conductive material. Theweight ratio of indium and bismuth to carbon can advantageously be inthe range of 0.10-1.50, e.g. about 30 wt. %.

Typically, the electrochemical reduction of carbon dioxide into chemicalreduction products is performed in an electrochemical cell orphotochemical cell having at least two cell compartments containing therespective electrodes. Carbon dioxide is supplied to the cathode. Thecathode is preferably a gas-diffusion electrode providing a high surfacearea or interface for solid-liquid-gas contact. Such a gas-diffusionelectrode comprises an electrically conductive substrate, which mayserve as a supporting structure for a gas-diffusion layer. Thegas-diffusion layer provides a thin porous structure or network e.g.made from carbon, for passing a gas like carbon dioxide from one side tothe other. Typically the structure is hydrophobic to distract water. Thegas-diffusion layer may comprise a catalytically active material.

Therefor a further aspect of the invention relates to a gas-diffusionelectrode, comprising a gas-diffusion layer on an electricallyconductive substrate, wherein the gas-diffusion layer comprises thecatalyst system according to the invention as outlined above. The binarymetal catalyst system of bismuth and indium according to the inventionmay be embedded in the gas-diffusion layer structure or provided as oneor more additional separate layers thereof. As explained above, aparticulate carbon is a preferred example of the conductive support forthe catalyst. The catalyst system is preferably bonded to theelectrically conductive substrate using a hydrophobic binder such asPTFE. Examples of suitable substrates include metal structures likeexpanded or woven metals, metal foams, and carbon structures includingwovens, cloth and paper.

Yet another aspect of the invention is an electrochemical cellcomprising at least one gas chamber and at least one liquid chamber,which chambers are separated by a gas-diffusion electrode according tothe invention.

Generally the reduction of carbon dioxide is performed in anelectrochemical cell, typically a divided cell having two cellcompartments. One cell compartment contains the anode, and the othercell compartment contains a gas-diffusion cathode electrode according tothe invention, comprising the binary metal electrocatalyst of bismuthand indium. The two cell compartments may be separated by a suitablemembrane, e.g. made from porous glass frit, microporous material, ionexchanging membrane or ion conducting bridge, allowing ionic species totravel from one compartment to the other, such as protons generated atthe anode to the cathode compartment.

A further aspect of the invention concerns a method of preparing agas-diffusion electrode as defined above, comprising the binary metalelectrocatalyst system according to the invention. This manufacturingmethod comprises a step of providing an electrically conductivesubstrate and a step of applying the catalyst system according to theinvention, comprising indium and bismuth and an electrically conductivesupport, in particular particulate support material, and a binder to thegas-diffusion layer of the gas-diffusion electrode.

The electrocatalyst loaded gas-diffusion electrode can be manufacturedin various ways including spraying, casting and sintering, often usingone or more suitable binders.

The invention also relates to a method of electrocatalyticallyconverting carbon dioxide into valuable products or productintermediates. This method comprises:

-   -   introducing an anolyte to a first cell compartment of an        electrochemical cell, the first cell compartment comprising an        anode;    -   introducing a catholyte and carbon dioxide to a second cell        compartment of the electrochemical cell, the second cell        compartment comprising a cathode, and applying an electrical        potential between the anode and the cathode sufficient to reduce        carbon dioxide to a reduced reaction product,    -   wherein the cathode comprises a catalyst system according to the        invention, in particular the cathode is a gas-diffusion        electrode according to the invention.

The method according to the invention allows to reduce carbon dioxide tocarboxylic acid and intermediates, including salts such as formate,glycolate, glyoxylate, oxalate and lactate, carboxylic acids, andglycols. The production of a carboxylic acid or carboxylic acidintermediate may be dependent on the pH of the electrolyte solution inthe cell, with lower pH ranges favoring carboxylic acid production. ThepH of the cathode compartment may be adjusted to favor production of oneof a carboxylic acid or carboxylic acid intermediate over production ofthe other, such as by introducing an acid (e.g., HCl or H₂SO₄) to thecathode compartment. The pH of the catholyte is preferably between about1 and 8. A pH range of 1-4 is preferable for production of carboxylicacids from carbon dioxide. A pH range of 4-8 is preferable forproduction of carboxylic acid intermediates from carbon dioxide.

The electrical potential may be a DC voltage. In preferred embodiments,the applied electrical potential is generally between about −1.5V vs.SCE and about −6V vs. SCE, preferably from about −1.5V vs. SCE to about−5V vs. SCE, such as in the range of −3V vs. SCE to −5V vs SCE and morepreferably from about −1.5V vs. SCE to about −4V vs. SCE.

High Faradaic yield and selectivity of the catalyst system according tothe invention for conversion of carbon dioxide into formate/formic acidhave been shown at the cathode according to the reactionCO₂+2H⁺+2e⁻→HCOOH, while at the anode water may be oxidized into oxygenand hydrogen ions according to 2H₂O→4H⁺+O₂+4e⁻.

The hydrogen ions pass through the ion exchange membrane from theanolyte compartment to the catholyte compartment in the electrochemicalcell.

The carbon dioxide conversion to formate/formic acid is typicallyperformed in an aqueous medium, wherein the CO₂ is bubbled through theaqueous medium or distributed to the gas-diffusion electrode, e.g. usingperculator systems.

Non-aqueous media may also be used, e.g. in the direct conversion ofcarbon dioxide to oxalic acid or oxalate.

A homogeneous heterocyclic catalyst may be added to the cathodecompartment of the cell containing the cathode. The homogeneousheterocyclic catalyst may include, for example, one or more of 4-hydroxypyridine, adenine, a heterocyclic amine containing sulfur, aheterocyclic amine containing oxygen, an azole, a benzimidazole, abipyridine, furan, an imidazole, an imidazole related species with atleast one five-member ring, an indole, a lutidine, methylimidazole, anoxazole, phenanthroline, pterin, pteridine, a pyridine, a pyridinerelated species with at least one six-member ring, pyrrole, quinoline,or a thiazole, and mixtures thereof. If present, the homogeneousheterocyclic catalyst is preferably present at a concentration ofbetween about 0.001M and about 1M, and more preferably between about0.01M and 0.5M.

The chemicals derived as reaction products from the directelectrochemical conversion according to the invention can be processedfurther into industrial products. E.g. oxalic acid can be used as astarting material for the production of ethylene glycol and/or glycine.See e.g. US2016/0017503. Hydrogen may be introduced to the carboxylicacid or carboxylic acid intermediate to produce a glycol or a carboxylicacid, respectively. Hydrogen may be derived from natural gas or water.

The invention is further illustrated by the attached drawings andexamples. In the drawings

FIG. 1 shows an embodiment of an electrochemical cell according to theinvention; and

FIG. 2 is an embodiment of a gas-diffusion electrode according to theinvention.

In FIG. 1 a block diagram of a system 100 is shown in accordance with anembodiment of the present invention. System 100 may be utilized forelectrochemical production of carboxylic acid intermediates, carboxylicacids, and glycols from carbon dioxide and water (and hydrogen forglycol production). The system 100 generally comprises anelectrochemical cell 102, a liquid source 104, an energy source 106, acarbon dioxide source 108, a product extractor 110 and an extractor 112,the latter in this embodiment for the recovery of oxygen produced at theanode. In an embodiment the liquid source 104 is a water source. Inanother embodiment the liquid source is an organic solvent source. Aproduct or product mixture may be obtained from the product extractor110 after extraction. An output gas containing oxygen may be output fromthe oxygen extractor 112 after extraction.

In the embodiment shown the cell 102 is a divided electrochemical cell.The cell 102 reduces carbon dioxide into products or productintermediates. The reduction may take place by introducing such asbubbling carbon dioxide into an electrolyte solution in the cell 102. Atthe cathode 120 comprising the catalyst system according to theinvention carbon dioxide is reduced into a carboxylic acid or acarboxylic acid intermediate.

The cell 102 generally comprises two or more or cell compartments 114 a,114 b, a separator 116 e.g. a ion exchange membrane, an anode 118 inanode cell compartment 114 a, and a cathode 120 in cathode cellcompartment 114 b on an opposite side of the separator 116. The cathode120 includes a catalyst system according to the invention suitable forthe reduction of carbon dioxide. An electrolyte solution e.g., anolyte122 a and catholyte 122 b may fill the respective cell compartments 114a and 114 b.

The liquid source 104 preferably includes a water source, such that theliquid source 104 may provide pure water to the cell 102. The liquidsource 104 may provide other fluids to the cell 102, including anorganic solvent, such as methanol, acetonitrile, and dimethylfuran. Theliquid source 104 may also provide a mixture of an organic solvent andwater to the cell 102.

The catholyte 122 may include an aromatic heterocyclic catalyst, e.g. ina concentration of about 10 mM to 1 M. The electrolyte may also includeone or more suitable salts, such as KCl, NaNO₃, Na₂SO₄, NaCL, NaF,NaClO₄, KClO₄, K₂SiO₃ or CaCl₂, e.g. at a concentration of about 0.5 M.

Other additives may include Group I cations (H, Ii, Na, K, Rb and Csexcept Fr), divalent cations (e.g., Ca²⁺, Mg²⁺, Zn²⁺) ammonium,alkylammonium cations and alkyl amines. Examples of anions comprisehalides, carbonates, bicarbonates, nitrates, nitrites, perchlorates,phosphates, polyphosphates, silicates and sulfates. .Bicarbonate is apreferred anion.

The pH of the cathode compartment 114 b is preferably between about 1and 8.

The energy source 106 may include a variable voltage source. The energysource 106 may be operational to generate an electrical potentialbetween the anode 118 and the cathode 120. The gas source 108 preferablyincludes a carbon dioxide source, such that the gas source 108 mayprovide carbon dioxide to the cell 102. E.g. the carbon dioxide isbubbled directly into the compartment 114 b containing the cathode 120.For instance, the compartment 114 b may include a carbon dioxide input,such as a port 126 a configured to be coupled between the carbon dioxidesource and the cathode 120.

The carbon dioxide may be obtained from any source, preferably arenewable source. The product extractor 110 may include an organicproduct and/or inorganic product extractor. The product extractor 110generally facilitates extraction of one or more products e.g.,carboxylic acid, and /or carboxylic acid intermediate from theelectrolyte 122. The extraction may occur via one or more of a solidsorbent, carbon dioxide-assisted solid sorbent, liquid-liquidextraction, nanofiltration, crystallization and electrodialysis. Theextracted products may be presented through a port 126 b of the system100 for subsequent storage, consumption, and/or processing by otherdevices and/or processes at A. In an embodiment the carboxylic acid orcarboxylic acid intermediate is continuously removed from the cell 102,where cell 102 operates on a continuous basis, such as through acontinuous flow-single pass reactor where fresh catholyte and carbondioxide is fed continuously as the input, and where the output from thereactor is continuously removed. In other embodiments, the carboxylicacid or carboxylic acid intermediate is continuously removed from thecatholyte 122 via one or more of adsorbing with a solid sorbent,liquid-liquid extraction, and electrodialysis.

The separated carboxylic acid or carboxylic acid intermediate may beplaced in contact with a hydrogen stream at A, e.g. in an additionalreactor, to produce a glycol or carboxylic acid, respectively.

Oxygen may be discharged from extractor 112 through port 128.

An embodiment of a gas-diffusion electrode according to the invention isshown in FIG. 2 . FIG. 2 represents a schematic illustration of anelectrochemical cell 200 utilizing an anode electrode 202 for the anodereaction, in this specific embodiment a hydrogen gas-diffusionelectrode, and a carbon dioxide gas-diffusion electrode 204 for thecathode reaction of reducing carbon dioxide e.g. to formate. The cathode204 may have a carbon dioxide internal gas plenum 206 in the currentcollector 208 of the electrode 204 to distribute carbon dioxide evenlyinto the gas-diffusion electrode. A cathode trickle bed solutiondistributor or percolator 210 is present in the catholyte cellcompartment 212. The catholyte solution may be introduced at the topentry 214 of the catholyte compartment 212 and the catholyte solution isdistributed evenly down the cell and is discharged via exit 216 at thebottom of the catholyte compartment 212. Alternatively, the flow may bereversed, so that the flow is in the upward vertical direction. Thesolution may be fed at specific rates, such as in the range of 0.001 to10 liters per minute or more depending on the electrochemical celldimensions, so that the cathode gas diffusion electrode 204 may not beflooded with the catholyte solution due to excessive pressure, and so asto maintain good ionic contact with the cathode gas diffusion electrode204 for the transfer of electrons into the solution in the reduction ofcarbon dioxide. The flow and pressure of the catholyte flow are suchthat minimal amounts of catholyte solution pass through the gasdiffusion electrode 204 into the carbon dioxide gas plenum 206 insidethe cathode current collector 208, and that the carbon dioxide gasreduction within the gas diffusion electrode is sufficient, so as toobtain a reasonable cathode current density, e.g. in the range of 10mA/cm² to 1000 mA/cm², or more preferably in a range of about 50 to 500mA/cm². An energy source (not shown) is operably coupled with theelectrodes 202 and 204 to reduce carbon dioxide at the cathode 204.Carbon dioxide is fed to the gas-diffusion electrode 204 via entry 218into the gas plenum 206. Micro-channels 220 may be provided to passcarbon dioxide from the plenum 206 to the gas-diffusion electrode 204that comprises the bismuth indium catalyst system. Carbon dioxide leavesthe cell through exit 222.

The anode side of the cell is similarly constructed. In this embodimenthydrogen gas is fed via entry 224 to gas plenum 226 provided withmicrochannels 228 and leaves the cell via exit 230. Anolyte isintroduced at entry 232, flows through a distributor 234 down to theexit 236. A ion exchange membrane 238 is arranged between the anolyteand catholyte distributors 234 and 210.

The cathode trickle bed 210 may include a thin construction, e.g. madefrom non-conductive corrosion resistant polymer plastics, such as PTFE,polypropylene and the like, in the form of screen-like or convolutedforms so to distribute the catholyte solution evenly as it passes downthe gas-diffusion electrode 204. Alternatively, the trickle bed materialmay include conductive carbon and graphite, or potentially bemanufactured from metal. The entry and exit ports of the catholytecompartment are designed such that the flow distribution of liquid isuniform along the cross section of the trickle bed at the top andbottom. In another embodiment the GDE cathode may be able to be operatedin a partially flooded or possibly fully flooded condition, and the flowconditions and electrolyte may be adjusted to operate the cathode inthis mode.

EXAMPLE 1 Screening Catalyst

Various binary metal catalysts were screened for their formate Faradaicyield in a test set up. The test set up comprised a 3 chambered glasscell wherein the electrodes were positioned. 0.75M KHCO3 was used aselektrolyt. Potentiostatic (xV vs SCE) electrolysis for theelectrochemical reduction of CO₂ to formate was performed during 3.5-5hrs. Tables 1 and 2 show the results. It has appeared that a 50 wt. % Bisample showed the best results in this screening test, while a 10 wt. %Bi sample outperformed a 90 wt. % Bi sample.

TABLE 1 Screening test results Formate Faradaic Catalyst E1/2 (V) vsSCE) Yield (%) In/Bi 50/50 −1.90 79.88 In/Bi 90/10 −1.90 76.81 AnodizedIn −1.75 75.73 In/Bi 10/90 −1.90 71.97 Bi/Pb 55.5/44.5 −1.90 70.87 Sn/Zn60/40 −1.90 57.81 In/Sn 70/30 −1.90 53.64 In/Zn 90/10 −1.90 51.81 Sn/Pb50/50 −1.90 48.42 In/Sn 30/70 −1.90 45.00 In/Sn 50/50 −1.90 41.46 In/Sn30/70 −1.60 30.24 In/Sn 96/4 −1.75 28.79 Au/Ni (82/18) −1.90  3.35 In−1.9  63.47

TABLE 2 Screening Test Results Formate Faradaic Alloy E1/2 (V) vs SCEYield (%) In/Sn 50:50 rod −1.46  7.69 −1.60 16.91 −1.90 54.21 Sn/Zn60/40 −1.90 57.81 Bi:Pb −1.60 18.25 −1.75 68.78 −1.90 70.87 Sn:Pb −1.7541.97 −1.90 48.42 In/Sn 70/30 −1.60 19.39 −1.75 46.87 −1.90 53.64 In/Sn30/70 −1.60 30.24 −1.75 51.88 −1.90 45.00 In/Sn 96/4 −1.60 27.28 −1.7528.79 In/Sn 50/50 −1.90 41.46 In/Bi 90/10 −1.75 82.26 −1.80 68.83 −1.9076.82 In/Bi 10/90 −1.75 57.53 −1.80 65.57 −1.90 71.97 In/Bi 50/50 −1.7573.70 −1.80 82.13 −1.90 79.88 In/Zn 90/10 −1.70 52.30 −1.80 59.64 −1.9051.81

EXAMPLE 2 Preparation of Binary Metal Catalyst System In/Bi on C

InCl₃, Bi(NO₃)₃*5H₂O and tri-sodium citrate dehydrate were weighted asshown in Table 3 and put inside a two-neck round bottom flask containing100 mL of tri-ethylene glycol and Vulcan carbon (available from Cabot).The round bottom flask was placed in an oil bath and fitted with acondenser. The system was continuously purged with N₂ gas. The oil bathtemperature was set to 100° C. The content of the flask was stirred.After the system reached the desired value of the temperature, it wasallowed to stabilize for about 10 minutes, before rapidly injecting awater solution of NaBH₄ using a syringe and needle. The NaBH₄ wasfreshly prepared and sonicated in order to speed up the solubilizationprocess. As soon as the NaBH₄ was injected, a vigorous bubbling wasobserved in the mixture. The color of the suspension was black and nochange in it was observed throughout the course of the reaction. Afterinjecting NaBH₄, the system was maintained at 100° C. under stirring for15 minutes. Then the heater was turned off and the suspension wasallowed to cool slowly. At room temperature the suspension wastransferred into 4 centrifuge tubes and centrifuged at 8000 rpm for 30min. The supernatant was poured out and ethanol was added into thetubes, followed by a thorough washing. The washing was performed bysonicating the suspension for 10 min. Then centrifugation at 8000 rpmfor 30 minutes was performed. This process was repeated 3 times. At theend ethanol (90 mL) was added into the tubes and the overall content wastransferred in a 100 mL glass jar. The resulting mixture was sonicatedfor 40 minutes at room temperature and then magnetically stirred for 15minutes. The thus obtained emulsion (catalyst ink) was ready for sprayapplication.

The In:Bi weight ratio in the thus prepared catalyst is 52.3:47.6.

TABLE 3 Material Mass (mg) InCl3 310 BiNO₃ × H₂O 340 Na₃Citrate 441Carbon 716 NaBH₄ 946

EXAMPLE 3 Preparation of Gas Diffusion Electrode (GDE)

A gas-diffusion electrode with a geometric surface area of about 172 cm²was cut using a metallic blade. The GDE thus prepared was fixed on analuminum panel using magnets and positioned at an angle of about 60°from the horizontal planed inside a ventilated fume hood. The catalystink was sprayed on the GDE using a manual air brusher at roomtemperature under atmospheric conditions.

1. A catalyst system for catalyzed electrochemical reactions, comprisinga catalyst, wherein the catalyst comprises 5-94 wt. % bismuth and 6-95wt. % indium, based on a total amount of bismuth and indium.
 2. Acatalyst system according to claim 1, further comprising an electricallyconductive support.
 3. A catalyst system according to claim 1, whereinthe amount of bismuth is in a range of 10-90 wt. %, based on the totalamount of bismuth and indium.
 4. A catalyst system according to claim 1,wherein the amount of bismuth is in a the range of 40-60 wt. %, based onthe total amount of bismuth and indium
 5. A catalyst system according toclaim 2, wherein the conductive support comprises a porous structure ofcarbon particles.
 6. A gas-diffusion electrode comprising: agas-diffusion layer on a conductive substrate, the gas-diffusion layercomprising a catalyst system for catalyzed electrochemical reactions,comprising a catalyst, wherein the catalyst comprises 5-94 wt. % bismuthand 6-95 wt. % indium, based on a total amount of bismuth and indium. 7.A gas-diffusion electrode according to claim 6, wherein the catalystsystem is bonded to the conductive substrate by a hydrophobic binder. 8.An electrochemical cell comprising: at least one gas chamber and atleast one liquid chamber, which chambers are separated by agas-diffusion electrode, the gas-diffusion electrode including agas-diffusion layer on a conductive substrate, the gas-diffusion layercomprising a catalyst system for catalyzed electrochemical reactions,the catalyst system comprising a catalyst, wherein the catalystcomprises 5-94 wt. % bismuth and 6-95 wt. % indium, based on a totalamount of bismuth and indium. 9-10. (canceled)
 11. A method ofelectrocatalytically reducing carbon dioxide, comprising introducing ananolyte to a first cell compartment of an electrochemical cell, thefirst cell compartment comprising an anode; introducing a catholyte andcarbon dioxide to a second cell compartment of the electrochemical cell,the second cell compartment comprising a cathode, and applying anelectrical potential between the anode and the cathode sufficient toreduce the carbon dioxide to a reduced reaction product, wherein thecathode comprises a catalyst system, the catalyst system for catalyzedelectrochemical reactions, comprising a catalyst, wherein the catalystcomprises 5-94 wt. % bismuth and 6-95 wt. % indium, based on a totalamount of bismuth and indium.
 12. A method according to claim 11,wherein the cathode is a gas-diffusion electrode, the gas-diffusionelectrode comprising: a gas-diffusion layer on a conductive substrate,the gas-diffusion layer including the catalyst system.
 13. A methodaccording to claim 11, wherein carbon dioxide is reduced to a reactionproduct selected from carboxylates and carboxylic acids.
 14. A methodaccording to claim 11, wherein carbon dioxide is reduced to formate orformic acid in an aqueous medium.
 15. A method according to claim 11,wherein carbon dioxide is reduced to oxalate or oxalic acid in anon-aqueous medium.